CED coating is a well-known process for coating electrically conductive, in particular, metallic substrates, in particular, for the production of corrosion-protective primer layers on metallic substrates, such as, for example, automotive bodies.
In CED coating, the substrates are coated in a conventional CED coating bath (CED coating composition, CED coating agent, CED coating paint), and excess, inadequately adhering CED coating composition is then rinsed from the substrates back into the CED coating tank. To avoid an increase in the volume of the CED coating bath, the substrates are conventionally rinsed with ultrafiltrate obtained from the CED coating bath.
The ultrafiltrate is obtained by known methods and, in addition to water, contains, for example, low molecular constituents, neutralizing agents, solvents and dissolved salts from the CED coating bath.
Finally, the substrates can be rinsed with water.
After rinsing, the CED coating layers are thermally cured (cross-linked) by baking in large, directly or indirectly heated circulating air ovens. Directly heated circulating air ovens are heated by combustion of, for example, natural gas, the combustion waste gases entering the circulating air of the oven. In circulating air ovens indirectly heated by means of heat exchangers, no combustion waste gases enter the circulating air of the oven.
It has been observed in industrial practice that the adhesion of subsequent coating layers, in particular underbody sealant layers, applied onto baked CED coating layers is often inadequate if the CED precoating has been baked in indirectly heated circulating air ovens.
WO 01/40550 solves the problem of inadequate adhesion of subsequent coating layers by a process for the production of an electrodeposition coating layer with improved adhesion towards subsequent coating layers by electrodeposition of a coating layer of an electrodeposition coating composition onto an electrically conductive substrate and baking, wherein the electrodeposition coating composition used contains one or more water-insoluble organic nitrites and/or nitrates and baking proceeds in an indirectly heated circulating air oven.
Metal compounds, for example, specific metal salts, are used in CED coating compositions, in particular, as cross-linking catalysts and/or anti-corrosion additives. In particular, lead compounds, such as lead silicate, often also in combination with tin compounds, such as dibutyl tin oxide, have been used. More recently, lead-free CED coating compositions have become known. CED coating compositions containing bismuth compounds have acquired particular significance, as described, for example, in U.S. Pat. No. 5,936,013, U.S. Pat. No. 5,554,700, U.S. Pat. No. 5,908,912, U.S. Pat. No. 6,174,422, U.S. Pat. No. 5,670,441, WO 96/10057, U.S. Pat. No. 5,972,189, WO 00/50522, U.S. Pat. No. 6,265,079, EP 1 041 125, WO 00/47642, WO 01/51570, U.S. Pat. No. 5,702,581 and U.S. Pat. No. 5,330,839.
Surprisingly, the solution to the problem disclosed in WO 01/40550 can equally well be achieved if, instead of the water-insoluble organic nitrites and/or nitrates disclosed therein, certain water-soluble metal nitrates are used as an additive in CED coating compositions and if a minimum proportion of the area of the substrate surface coated with a CED coating layer from the CED coating compositions is not rinsed with water prior to thermal curing.
Nitrate in anionic form cannot be deposited on the cathode; it is in particular also consequently surprising that the same effect as described in WO 01/40550 can be achieved.
Advantages are achieved not only in terms of the simpler handling of water-soluble metal nitrates in comparison with the handling of the water-insoluble organic nitrites and/or nitrates, but also in that a metal required as a formulation constituent can also be introduced into the CED coating composition by means of a single chemical compound, namely, a metal nitrate.